Dear trickyt57-clive
Thanks for using Microsoft products and posting in the community.
I understand that your current situation is that you want to change the default language of Word on your Mac to English, but I wonder if you are concerned about your system language being changed, as you mentioned that your documents still appear in French even though they have been changed to English. I am not sure which language you are used to.
In your case, I have tested this and it is not possible to change the language of an Office application on a Mac alone, you need to change your system language before Word will change to what you want.
You can through the Settings---General---Language&Region---Drag the language you want to use to the top---Choose to log out of your current system account---Log in again that language change.

I have also found relevant links for you from which you can getmore detailed information:
https://support.apple.com/guide/mac-help/change-language-region-settings-on-mac-intl163/mac
Disclaimer: This is a non-Microsoft website. The page appears to be providing accurate and safe information. Watch out for ads on the site that may advertise products frequently classified as PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
If you have any additional questions or need further assistance, please feel free to contact us.
Thank you again for your patience and understanding!
Have a nice day!
Best Regards,
Maxine | Microsoft Community Support Specialist